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Venus sun transits were historically of huge interest to astronomers in the
days of sail who wanted to provide optimum celestial data for navigators.
This time around the main scientific tasks of professional navigators will
be to learn more about the black-drop effect that frustrated previous
expeditions, and maybe to test some methods for looking for new planets.
This tranist will be interesting to laypersons who can get a kick out of a
rare astronomical event, and relive a phenomenon that our predecessors took
very seriously indeed.

This email contains a proposal and question about what we navigators can
learn from this transit, a question about using a sextant to view the June 8
transit, some information about the transit's historical value, and some
tips on viewing the transit.

1. Value to modern navigators:

Celestial navigators might get a lot of value from observing this rare
transit of a "large" planetary image.  At very least it is a way of testing
our ability to view Venus' disk (1/30 the size of the sun's on that day)
with our sextants' telescopes, for example.  That's all I plan to do.  Do
you have other suggestions?

2. The Question:

I am gearing up to check out Venus' transit of the sun at daybreak on June
8.  I should have a great view from our back porch on Prince Edward Island,
if the clouds part.  I plan to view the transit using the 6x30 telescope on
my sextant, with the appropriate horizon and index mirror shades.  That
shouldn't be a problem, should it?  Even if I watch for long periods of
time?  The shades are in front of the mirrors and telescope, so they are not
exposed to concentrated rays of sunlight as they would be if the observer
makes the terrible mistake of putting a shade between his eye and the
aperture of a telescope.  The shade could heat up and shatter if it is
placed between eye and aperture, suddenly flashing concentrated light into
the observer's eye with terrible consequences.

3. The Information:

This month's Astronomy magazine has some great articles about Venus' sun
transits, especially one by Ray Jayawardhana.  Some bare bones:

- Venus gets between earth and the sun every 121.5 yrs, 8 yrs, 105.5 yrs, 8
yrs and then the cycle repeats that way.
- In 1629 Keppler predicted the 1631 Venus transit, but did not live to see
it.  Nor did anyone else document an attempt.
- Two English friends carefully documented the 1639 transit by
simultaneously casting the sun's image from a telescope onto paper, in two
places 30 miles apart.
- In 1677 Halley echoed an earlier Scottish mathematician by proposing a
method for estimating the distance of the sun from the earth using planetary
sun transits.
- In 1761, despite the Seven Year's War between England and France,
astronomers in those two countries and Austria coordinated expeditions of
120 observers sent to 5 far-flung locations on sailing ships.  They tried to
time Venus' transit simultaneously.  They were not particularly successful.
- In 1769 they tried again, sending observers even farther afield.  This
time Cook was dispatched to Tahiti to make careful observations of the Venus
transit (and to secretly look for Australia).  His was the longest trip
among those who made expeditions.  Cook had a cloudless day for the
observation, but was frustrated by the black-drop effect, which made it to
hard to see exactly when Venus entered and left the sun's margins.  However
when compared with other expeditions, the various expeditions' combined
results gave estimates of earth-sun distance accurate to within a few
percent of the modern value.
- In 1874 seven nations launched 60 expeditions to nail down that estimate
using Venus' second transit of the century.  The issue was so important to
navigation that Congress appropriated $177,000!  Unfortunately clouds
frustrated several groups, and the data collected did not improve the
measurement.
- In 1882 they tried again, encountering better weather.  It took years to
analyze the data, but the best estimate was not highly regarded by the
astronomical community.  Better estimates of AU were available from other
methods.

That was the last Venus sun transit before the one we are about to see on
June 8.

4. Viewing tips:

Venus will enter the lower left margin of the sun, travel right in a
downward slope across the lower 1/5th of the sun's face, and exit to the
lower right.  Use this table to determine whether you will be able to see
it:

~0520 UT: Venus first touches sun's margin.
~0820 UT: mid-transit.
~1110 UT: Venus exits the face of the sun.

If you miss this transit, then you can try for the next one in 2012.  But if
you miss that one, it will be up to our descendants in the 22nd century to
give it a try.

For more detailed information:
http://sunearth.gsfc.nasa.gov/eclipse/transit/TV2004.html
which contains this useful viewing map:
http://sunearth.gsfc.nasa.gov/eclipse/transit/TV2004/TV2004-Map1a.GIF

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
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